This invention relates to radiographic apparatus and procedures and more particularly to scanning X-ray systems which produce signals which may be used to present a visible image at a cathode ray tube screen or the like.
The present invention was initially developed for usage in dental and medical radiology, and to facilitate description the invention will be herein discussed with reference to this particular field of use. As will be apparent, the apparatus may also be advantageously adapted to various other radiographic operations.
The procedure which has been most commonly used by dentists to obtain radiographic images of the teeth, jaw structure or the like of patients produces what is called a periapical X-ray radiograph. In this known procedure an X-ray film packet is inserted into the patient's mouth near the teeth or other anatomical structure to be imaged. The procedure makes use of an X-ray tube of the form which generates X-rays at a fixed point on an anode and in which the X-rays radiate out from that fixed point. The tube is provided with a shield cone which is directed at the film packet through the teeth or other structures to be imaged. Precise positioning of both the film packet and the shield cone is necessary to obtain useful X-ray images.
Although the conventional periapical X-ray procedure is very extensively used, it is subject to several serious disadvantages. The need to insert and retain a relatively large film packet in a patient's mouth, for example, often causes discomfort or gagging and may not be tolerable to certain patients such as small children and elderly persons. Further, no visible image is available to the dentist until the film packet has been removed and subjected to time-consuming development procedures. An instantaneous radiographic image can be much more useful to the dentist.
Another very serious problem is that undesirably large radiation dosage exposures of the patient are needed in order to produce a complete set of dental X-ray images. This is in part a result of the very low detection efficiency of the unscreened X-ray film commonly used for dental X-ray operations.
Techniques for reducing radiation exposure have heretofore been developed utilizing screened film in which detection efficiency is greatly improved by disposing an image-intensifying phosphorescent material in contact with the X-ray film emulsion surface. Owing to several disadvantages of its own, such as reduced image definition, the screened film procedure has not proved to be practical in many situations.
The problem of high radiation exposure in dental radiology is often aggravated by a need to repeat the X-ray imaging process. It may be found that the critical alignment requirements were not met during the original exposure or errors in developing the exposed X-ray film may be made, both of which are fairly common occurrences.
In part to alleviate the radiation exposure problem, another procedure known as the pantomographic image technique has been developed and has been extensively utilized in the recent past by dentists and oral surgeons. In this procedure panoramic or wide angle X-ray images are produced by generating a narrow linear X-ray beam which is revolved during the exposure about an axis of rotation situated within the patient's head. The X-ray tube is essentially a conventional one at which X-rays are generated at a small fixed point on an anode. Radiation generated at this point is collimated by a first slit which is parallel to the axis of rotation and then passes through the patient's head and then through a second similar collimating slit situated in front of a screened film cassette which is rotated in synchronism with the rotational movement of the X-ray beam. The tube and detector motion cases the X-ray beam to sweep across the intervening anatomical structures. Upon development of the film, a panoramic two-dimensional strip image is produced of curved anatomical structures in the patient's head such as the mandible or maxilla.
Although a significant reduction of patient radiation dosage may be realized in comparison with periapical procedures, the conventional pantomographic image technique is itself subject to several disadvantages. It is necessary that the X-ray beam pass through the entire skull of the patient, even if it is only desired to obtain an image of a portion of the skull such as the dental arch. Consequently, unwanted images are superimposed upon the desired image data. This makes interpretation of the image more difficult and detracts from the general quality of the image by obscuring desired data to some extent with undesired information. Moreover, radiation exposure remains undesirably high as the X-ray beam must necessarily penetrate through the entire skull. Anatomical structures which are not of particular interest are thereby necessarily subjected to radiation dosage which does not contribute any useful information but instead detracts from the quality of the desired data. Further a significant amount of X-ray scattering occurs during passage of the X-ray beam through the patient's entire head creating a background fog in the image on the developed film which undesirably limits the range of contrast in the image and which may cause loss of definition.
Additional losses of definition and contrast arise from the presence of the intensifying screen in front of the X-ray film. Underlying and supplementing these contrast limitations peculiar to the pantomographic image technique is the undesirably limited grey scale latitude of X-ray film in general. Still further, a long exposure time, typically about 20 seconds, is needed to complete a full mouth pantomographic image. As a result, problems often arise from patient motion or equipment vibration with consequent blurring of the resulting X-ray images. This tends to be particularly severe when the patient is an infant or young child. Finally, a considerable degree of distortion of the depicted objects is normally present in the conventional pantomographic image.
A radically different form of radiographic imaging system that alleviates or eliminates much of the disadvantages of prior techniques and apparatus is disclosed in Applicant's U.S. Pat. No. 3,949,229 and Applicant's copending application Ser. No. 663,988 filed Mar. 4, 1976. Applicant's copending applications Ser. No. 674,059, filed Apr. 5, 1976, and Ser. No. 673,908, also filed Mar. 4, 1976 are also directed to scanning X-ray systems of this general kind.
The general form of scanning X-ray system disclosed in the above-identified prior patent and copending applications dispenses with the use of film as an X-ray detection medium and produces signals which may be used to produce a visible image on the screen of a cathode ray tube display device including instantaneous images if desired. Radiation dosage of the patient is substantially reduced. The system may be utilized to image only a selected portion of a subject such as a patient's dental arch for example without including superimposed data from other regions of the subject. Image data may be electronically stored on magnetic tape or by any of various other data storage means and the image data may also readily be processed by various electronic enhancement techniques to further improve image quality or to emphasize specific image characteristics.
A system of the general type described in the above-described patent and copending applications uses a scanning X-ray tube in which an electron beam is systematically swept in a raster pattern on a broad target or anode plate to produce a moving point source of X-rays. The region of the subject which is to be imaged is situated between the anode plate of the X-ray tube and an X-ray detector which is small in relation to the size of the raster pattern and which may therefore readily be situated in the oral cavity or the like of a dental or medical patient or in similarly constricted interior spaces of an inanimate subject. The raster sweep signals of a cathode ray tube display are coordinated with the scanning action of the electron beam in the X-ray tube and a signal derived from the X-ray detector output is applied to the intensity signal terminal of the cathode ray tube. As a result, a visible radiographic image of the region of the subject situated between the X-ray source and the detector is produced on the screen of the display device.
In order to be most useful for dental and medical usages and for certain other radiological operations where similar problems may be encountered, a scanning X-ray system of this general type should possess certain specific capabilities. First, radiation dosage should be minimized to the extent possible while producing an image of high definition and contrast range. Second, the dentist or other operator should be able to position the X-ray detector very precisely relative to the X-ray tube at any of a plurality of different positions within the patient's mouth, or in other constricted spaces, with a minimum of difficulty and with maximum patient comfort.
Further, it is highly desirable that the effective focal length of the system be readily and precisely changeable in order to obtain images of different degrees of magnification.
Still further, such a system should minimize optican distortions and other forms of image degradation, which can be present in apparatus of this general form, in order to facilitate image interpretation.